Damien4703,
I'm glad that you feel ready to take some action. Please remember that the process may start to feel overwhelming and you may fall back into old patterns and habits periodically. You may feel like "it's not working" and need encouragement to recognize small advances, or just keep plowing on, even when there is no outward evidence of change. It's okay to feel like throwing in the towel and to verbalize that here.
You've mentioned several areas that could be addressed with some boundaries, so that might be a good place to start. I suggest reading the resources around boundaries on this site:
https://bpdfamily.com/content/setting-boundariesSelecting the right boundary to start with is a personal decision. You can look for an area that causes you the most anxiety and will thus provide the most relief, but those may be a bit big to take on at first. I would suggest something that is well defined and "managable" that you can be consistent in enforcing without to much effort.
From the things that you have mentioned, I think (but correct me if I'm wrong) that setting a boundary on her access to your phone might be a place to start. This is a relatively easy boundary to set - change your passcode and don't give her the new one. Reinforcing will take some skills. You may want to be prepared with a "scripted" response for the day that she tries to access your phone and can't (if you choose not to inform her of the boundary up front). This is a good chance to practice SET (Support, Empathy, Truth). It might sound something like:
"I really want us to have a relationship based on mutual trust" (Support)
"I understand that you are afraid that I am hiding another relationship from you, and that fear drives you to look for evidence on my phone" (Empathy)
"Checking my phone for that evidence stands in the way of real trust. I have given you no reason to distrust me, and the constant search for evidence of unfaithfulness on my phone is a reminder to me that you don't trust me. You will no longer have access to my phone, and I would like you to look for other ways to address your fears." (Truth)
You should be prepared to repeat this script or some portion of it EVERY time she begs, rages, cries, threatens, etc in an attempt for you to give her the passcode. You should also be prepared to say "I've already explained why you no longer have access to my phone. I'm not willing to discuss it any further. I will be xyz - feel free to text me when you are ready to do/talk about something else." Be ready to enforce the boundary through whatever she throws at you, up to and including the end of the relationship. Know that doing so will teach both of you that you are a man of your word who can and will choose what is acceptable in a relationship and what is not.
Does that sound like something you could do? Is there a different boundary that you would like to start with? You may want to read recent posts started by DaddyBear77. He is also working on boundaries with his wife and had some success in defending the boundary of not being expected to wake up in the middle of the night and soothe/argue with her.
Hang in there and keep posting.
BeagleGirl